1. Field of the Invention
The present invention relates to an isolated converter for use in a switching power supply or the like.
2. Description of the Related Art
An isolated converter (such as a DC-DC converter and isolated AC-DC converter) used in a switching power supply or the like is formed of a transformer, a primary-side circuit connected to a primary coil of the transformer, and a secondary-side circuit connected to a secondary coil of the transformer such that power supplied to the primary-side circuit is transformed in terms of voltage by the transformer and a resultant voltage is output from the secondary-side circuit.
In some cases, when the isolated converter is used in a device which needs to be small in size, a multilayer sheet transformer is employed as the transformer as will be described below. FIG. 4 illustrates, in the form of an exploded view, a multilayer sheet transformer. FIG. 5 is a cross-sectional view of the multilayer sheet transformer taken along line Axe2x80x94A of FIG. 4, and FIG. 6 is a cross-sectional view of the multilayer sheet transformer taken along line Bxe2x80x94B of FIG. 4.
As shown in FIG. 4, the multilayer sheet transformer 1 is formed integrally with a multilayer circuit board 4 on which a primary-side circuit 2 and a secondary-side circuit 3 are formed. The multilayer sheet transformer 1 includes coil patterns 6 (6a, 6b) formed on a plurality of sheet substrates shown in FIG. 6 (three sheet substrates 5a, 5b, and 5c in the example shown in FIG. 6) forming the multilayer circuit board 4, a core member 11 (11a, 11b) which is E-shaped in cross section as shown in FIG. 4, and a core combining member 13.
In the conventional technique, to achieve good electrical and magnetic characteristics such as conversion efficiency or the degree of coupling between the primary coil and the secondary coil of the transformer, coil patterns 6a of the primary coil and coil patterns 6b of the secondary coil are alternately placed in successive layers in the multilayer structure (hereinafter, such a multilayer structure of coil patterns 6 will be referred to as a sandwich structure). More specifically, as shown in FIG. 6, one set of the primary or secondary coil patterns 6 (the primary coil pattern 6a in the specific example shown in FIG. 6) is formed on the upper surface of each of sheet substrates 5a, 5b, and 5c, and the other set of the primary or secondary coil patterns 6 (the secondary coil pattern 6b in the specific example shown in FIG. 6) is formed on the lower surface of each of sheet substrates 5a, 5b, and 5c. 
These coil patterns 6 (6a, 6b) are coaxially disposed in the respective layers such that their central axes become coincident with each other so as to form a coil pattern unit 7. The plurality of primary coil patterns 6a formed on the respective sheet substrates 5a, 5b, and 5c are electrically connected to each other via connection conductors 15 (15a, 15b) extending via through-holes 14 (14a, 14b) so as to form the primary coil. Similarly, the plurality of primary coil patterns 6b are electrically connected to each other via connection conductors 17 (17a, 17b) extending via through-holes 16 (16a, 16b) so as to form the secondary coil.
In FIG. 6, reference numerals 8 denote insulating sheets (such as prepreg) disposed between sheet substrates to insulate the coil patterns 6a and 6b at vertically adjacent locations from each other.
As shown in FIG. 4, the multilayer circuit board 4 has core leg holes 10 formed at the center of the coil pattern unit 7 formed of the coil patterns 6a and 6b and at locations outside the coil pattern unit 7. Core legs 12 of the E-shaped core members 11a and 11b are inserted into the corresponding core leg holes 10 from the upper and lower sides of the multilayer circuit board 4 such that end faces of the respective core legs come into direct contact with each other as shown in FIG. 5. A pair of E-shaped core members 11a and 11b in contact with each other is fitted in a core combining member 13 having a shape shown in FIG. 4 such that the E-shaped core members 11a and 11b are combined together by the core combining member 13 and such that the coil pattern unit 7 is partially sandwiched by the respective E-shaped core members 11a and 11b inserted from the upper and lower sides of the multilayer circuit board 4, as shown in FIG. 5.
As described above, the multilayer sheet transformer 1 is formed integrally with the multilayer circuit board 4. Use of the multilayer sheet transformer 1 formed in such a manner allows a reduction in the thickness of the isolated converter.
In the conventional structure described above employing the multilayer sheet transformer 1, although it is easy to reduce the thickness of the isolated converter, it is difficult to reduce the size of the multilayer circuit board 4 and thus the total size of the isolated converter, for the reason described below.
That is, in the conventional structure, because the multilayer structure of the coil patterns 6 is obtained by means of disposing the coil patterns 6 into a sandwiched form as shown in FIG. 6, the coil pattern 6 (6a) formed on the outer surface of the top layer of the multilayer circuit board 4 is a coil pattern of the primary coil, while the coil pattern 6 (6b) formed on the outer surface of the bottom layer of the multilayer circuit board 4 is a coil pattern of the secondary coil. That is, the coil patterns formed on the outer surfaces of the top and bottom layers of the multilayer sheet transformer 1 are for different coils on either primary or secondary sides. Therefore, when a very large overvoltage appears in either the primary-side circuit 2 or the secondary-side circuit 3 for some reason, the overvoltage tends to create a spark, along the surface of the E-shaped core members 11a and 11b formed of ferrite or the like of the multilayer sheet transformer 1, between the primary-side circuit 2 and the secondary-side circuit 3, thereby causing an electrical breakdown between the primary and secondary circuits.
More specifically, a spark is easily created by an overvoltage between the primary-side circuit 2 and the surface of the E-shaped core member 11a close to the coil pattern 6a of the primary coil. Similarly, a spark is easily created by an overvoltage between the secondary-side circuit 3 and the surface of the E-shaped core member 11b close to the coil pattern 6b of the primary coil. If an overvoltage occurs, for example, in the primary-side circuit 2, the overvoltage first creates a spark between the primary-side circuit 2 and the surface of the E-shaped core member 11a disposed on the upper side. The overvoltage then propagates to the E-shaped core member 11b disposed on the lower side and creates a spark between the secondary-side circuit 3 and the surface of the E-shaped core member 11b located on the lower side. As a result, an electrical breakdown occurs between the primary and secondary circuits.
To ensure that no electrical breakdown occurs between the primary and secondary circuits, it is required that the multilayer sheet transformer 1 and the secondary-side circuit 3 be spaced from each other by a large enough distance to prevent an electrical breakdown between the primary and secondary sides. Because of the necessity of the large isolation space between the primary and secondary sides of the multilayer sheet transformer 1, it is difficult to reduce the size of the multilayer circuit board 4 and thus the total size of the isolated converter.
Further, as described earlier, because the coil patterns 6 have to be disposed so as to obtain the sandwich structure, the coil patterns 6a formed on the respective sheet substrates 5 have to be connected to each other by the connection conductors 15 (15a, 15b) via the through-holes 14 (14a, 14b) to obtain the primary coil, and the coil patterns 6b formed on the respective sheet substrates 5 have to be connected to each other by the connection conductors 17 (17a, 17b) via the through-holes 16 (16a, 16b) to obtain the secondary coil. Furthermore, it is necessary to form through-hole lands 20 on the upper and lower surfaces of the multilayer circuit board 4 so as to cover the openings of the through-holes 14 and 16.
The through-holes 14 and 16 and the connection conductors 15 and 17 for electrically connecting the coil patterns formed on the respective sheet substrates 5 and the through-hole lands 20 have to be formed such that they do not hinder the coil patterns 6a and 6b from being formed and such that good electrical isolation can be obtained. To meet the above requirements, the sheet substrates need to have a special space to dispose the through-holes 14 and 16, the connection conductors 15 and 17, and the through-hole lands 20. This also makes it difficult to reduce the size of the multilayer circuit board 4 and the total size of the isolated converter.
Another problem in the conventional structure described above is that the coil pattern 6a on the outer surface of the top layer and the coil pattern 6b on the outer surface of the bottom layer may e electrically connected to each other via the E-shaped core members 11a and 11b and thus the primary coil and the secondary coil may be electrically connected to each other.
In view of the above problems, it is an object of the present invention to provide an isolated converter which has excellent electrical isolation between primary and secondary sides and which can be easily formed so as to have a small size.
According to a first aspect of the present invention, to achieve the above and other objects, there is provided an isolated converter comprising a transformer of the type comprising a multilayer sheet transformer comprising a multilayer circuit board comprising a plurality of sheet substrates, a coil pattern forming a primary coil and a coil pattern forming a secondary coil disposed coaxially on the sheet substrates, and a core member disposed in a coil pattern unit formed of the coil patterns, the multilayer circuit board including a first area where a primary-side circuit on the side of the primary coil of the multilayer sheet transformer is formed, a second area where the multilayer sheet transformer is formed, and a third area where a secondary-side circuit on the side of the secondary coil of the multilayer sheet transformer is formed; the areas being located in order; a coil pattern formed on an outer surface of a top layer and a coil pattern formed on an outer surface of a bottom layer are for the same coil on either the primary or secondary side, wherein, in the case where the coil patterns formed on the outer surfaces of the top and bottom layers are for the primary coil, the multilayer sheet transformer is regarded as a part of the primary-side circuit and an insulation gap for achieving an electrical isolation between the primary and secondary sides is formed between the multilayer sheet transformer and the third area in which the secondary-side circuit is formed, while in the case where the coil patterns formed on the outer surfaces of the top and bottom layers are for the secondary coil, the multilayer sheet transformer is regarded as a part of the secondary-side circuit and an isolation gap for achieving an electrical isolation between the primary and secondary sides is formed between the multilayer sheet transformer and the first area in which the primary-side circuit is formed.
Preferably, in this isolated converter according to the present invention, coil patterns are formed on both surfaces of each sheet substrate such that a coil pattern formed on one surface of each sheet substrate and a coil pattern formed on the opposite surface of that sheet substrate are for the same coil on either the primary or secondary side, and wherein sheet substrates are disposed into a multilayer structure such that a primary-side sheet substrate on both surfaces of which coil patterns for the primary coil are formed and a secondary-side sheet substrate on both surfaces of which coil patterns for the secondary coil are alternately located.
In the multilayer sheet transformer constructed in the above-described manner according to the present invention, in the case where the coil patterns formed on the outer surfaces of the top and bottom layers are both for the primary coil, when an overvoltage appears in the circuit on the primary side, even if the overvoltage creates a spark between the circuit on the primary side and the core member of the multilayer sheet transformer, the overvoltage hardly creates a spark between the core member of the multilayer sheet transformer and the circuit on the secondary side, because the coil patterns formed on the outer surfaces of the top and bottom layers of the multilayer sheet transformer are not for the secondary coil but for the primary coil. Furthermore, because the multilayer sheet transformer and the circuit on the secondary side are spaced by the isolation gap, when an overvoltage appears in the circuit on the primary side, the overvoltage hardly causes a spark between the circuit on the secondary side and the circuit on the primary side via the surface of the core member. Thus, it is possible to prevent an electrical breakdown between the primary and secondary sides.
Conversely, when an overvoltage appears in the circuit on the secondary side, the overvoltage hardly creates a spark between the circuit on the secondary side and the core member of the multilayer sheet transformer, because the coil patterns formed on the outer surfaces of the top and bottom layers are both for the primary coil and because the multilayer sheet transformer and the circuit on the secondary side are spaced by the isolation gap. Also in this case, it is possible to prevent an electrical breakdown between the primary and secondary sides.
In the present invention, as described above, an electrical breakdown between the primary and secondary sides is prevented in a highly reliable fashion, and the isolation gap is needed only on one of the primary and secondary sides of the multilayer sheet transformer. Thus, in the present invention, in contrast to the conventional structure in which isolation gaps are needed on both primary and secondary sides of the multilayer sheet transformer, it is possible to reduce the size of the multilayer circuit board and thus the size of the isolated converter.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.